Processes for forming printed circuit boards are well known, and such boards are known to commonly include dielectric sheets, or substrates, the surfaces of which have electrically conductive layers adhered thereto, commonly utilizing adhesives, prior to forming, from such layers of conductive material, the required pattern of conductors, commonly known as "traces".
It is also known that the surfaces of the walls of holes in printed circuit boards can be plated during deposit of conductive material on other surfaces of a substrate. In so doing, it has been common to initially deposit conductive material on the substrate after drilling of holes in the board, which results in plating through the holes. The conductive pattern was then normally thereafter formed, as for example, by imaging using photographic techniques, applying resists selective to the imaging, and then etching away the unwanted portions of the conductive layers on the substrate.
In electroplating copper onto a substrate, it has heretofore been common to immerse the board in a high throwing power acid-copper sulfate bath (rather than in a standard composition acid-copper sulfate bath which has not been found to be acceptable for forming printed circuit boards). While a layer of copper has thus been deposited on the substrate, it has been found that such a layer often does not adhere to the substrate in an acceptable manner.
The problem of adhering a conductive layer to a dielectric substrate is discussed, for example, in U.S. Pat. No. 3,293,109, wherein it was suggested that improved bonding could be achieved using copper-copper oxide particles deposited in random clusters as a surmounting strata.
Sputtering of conductive material onto a dielectric material is also known, and such deposition is described, for example, in Thin Film Process, 1978, Academic Press, Inc., Chapter 11-4, Robert K. Watts. Sputtering has heretofore been used in connection with application for industrial coatings, thin film electronics, integrated circuits, print heads and other miscellaneous applications.
Processes for forming printed circuit boards using successive steps for deposition of conductive materials are shown, for example, in U.S. Pat. No. 3,294,654. In addition, processes for forming printed circuit boards utilizing successive sputtering steps for conductive layer deposition is shown, for example, in U.S. Pat. Nos. 4,077,854 and 4,455,181, and electro-deposit of a conductive material over a sputtered film is also shown, for example, in U.S. Pat. No. 4,077,854.
The use of a thin film barrier layer between a substrate and a vacuum-deposited metallic layer, with the metallic layer then having a further metallic layer electroplated thereon, is shown, for example, in U.S. Pat. No. 4,153,518, and stressing of individual layers on a substrate is shown, for example, in U.S. Pat. No. 2,984,595, with deposition of a stress free elecrtoless copper deposit being shown, for example, in U.S. Pat. Nos. 4,167,601 and 4,228,213.
Thus, while many different processes have heretofore been suggested for depositing conductive layers onto a substrate in forming a printed circuit board, there still often remains the problem of satisfactory adherence of such layers to the substrate.